This is a resubmission of Preoperative Lifestyle Intervention in Bariatric Surgery (1 R01 DK077102- 01). Although bariatric surgery is associated with weight loss and improvements in obesity-related comorbidities and psychosocial functioning, a significant minority of patients experience complications and poor outcomes. In light of the sharply increasing number of bariatric surgeries performed each year, efforts to optimize patient outcomes will have a major public health impact. There are several initiatives to establish best practices in bariatric surgery, with growing expert consensus that patients should lose weight prior to bariatric surgery in order to reduce surgical risks and improve outcomes. However, data to support this recommendation are lacking, and it is not known whether requiring preoperative lifestyle intervention for weight loss improves postoperative outcomes, or poses an unnecessary barrier to the surgical treatment of obesity. Accordingly, we propose a randomized, controlled trial to evaluate the impact of a preoperative lifestyle intervention that targets diet, exercise, and preparation for surgery as an adjunct to the surgical treatment of obesity. Patients will be randomized to a 6-month lifestyle intervention (n = 100) or to usual care (n = 100) prior to undergoing gastric bypass surgery. We aim to evaluate the effect of the intervention on select pre- and postoperative outcomes. We hypothesize that patients who participate in the preoperative intervention will exhibit greater improvements in weight and related outcomes and better preparation for surgery than those who receive usual care. After operation, we hypothesize that patients who participate in the intervention will exhibit better compliance and fewer behavior-related eating problems, as well as a lower rate of complications and fewer outpatient visits with surgery-related conditions than those who received usual preoperative care. Our secondary aim is to determine whether the intervention affects weight/BMI trajectory through 24 months after operation. The proposed investigation is significant as the first effort to systematically establish the impact of a preoperative behavioral lifestyle intervention in bariatric surgery. Study results will have a direct and immediate impact on current efforts to develop evidence-based best practices in bariatric surgery.